1. Field of the Invention
This invention relates to a method of hydrolyzing urea contained in aqueous solutions, such as waste-water streams. In particular, the present invention relates to a novel catalytic process for hydrolyzing urea.
2. Description of the Prior Art
Urea is commonly made today by reacting ammonia and carbon dioxide under elevated pressures and temperatures. This reaction also forms water as a by-product. After the reaction is over, the urea is usually separated from the reaction mixture by conventional concentration or crystallization methods. Thus, a urea product, normally in solid form, is recovered. The by-product water is also collected in various waste-water streams. Such streams still may contain a minor amount of urea, urea-related compounds and/or unreacted ammonia and carbon dioxide. Therefore, in nearly all commercial urea processes, these streams, before discharged as effluents, are passed through an ammonia desorber or stripper which recovers unreacted ammonia from the waste-water streams for re-use in the urea process or other chemical processes. However, when these waste-water streams are passed through the ammonia desorber, very little urea contained therein is affected by the stripping operation and, thus, substantially all of the urea in the waste-water streams is disposed of in the effluent streams.
It is now recognized by both the urea industry and the responsible governmental agencies that having urea-containing effluent streams may create possible environmental hazards. Therefore, several processes have been suggested for treating these urea-containing streams. For example, it is known that one of the most effective ways of removing urea from a water stream is simply to hydrolyze it back to ammonia and carbon dioxide at above ambient temperatures (the reverse reaction of the above-described urea synthesis). Some hydrolysis treatments have been disclosed. See U.S. Pat. Nos. 3,826,815, issued to Mavrovic on July 30, 1974 and 3,922,222, issued to Van Moorsel on Nov. 25, 1975. However, these hydrolysis treatments of urea have certain disadvantages. Most importantly, they require the construction of additional equipment for the hydrolysis. Also, this additional equipment must be constructed of expensive stainless steel because urea solutions may cause corrosion of ordinary carbon steel equipment. Still further, hydrolysis processes such as those illustrated in the cited patents require substantial energy costs because large amounts of steam are normally required to heat the urea hydrolysis reaction.
As described in U.S. Pat. application Ser. No. 823,883, now U.S. Pat. No. 4,087,513, filed on Aug. 12, 1977, the present inventor has found that it would be quite advantageous to utilize the CO.sub.2 recovery streams of adjacent ammonia production plants to hydrolyze the urea contained in waste-water streams formed in the production of urea. Specifically, that invention was found to be superior to the known urea hydrolysis processes because no new equipment was necessary and it was also possible to lower the energy requirements of the ammonia process.